ASD and anxiety and often co morbid.
You say the Psychiatrist said that many mothers describe the feeling of claustrophobia as being in an abusive relationship, where they are coercively controlled and when we fight it ie insist on spending time with other kids or friends, they react with anger and punish through aggression and spammimg with texts and calls - but you do understand that while it might feel like this, this is absolutely not what is going on, right? The Psyche did continue their point and make that clear right? I hope so!
What is going on here is that you have an absolutely terrified child. The world makes no sense to him and he is so scared out of his wits that he holds on as tightly as he can to the only thing that makes him feel at all safe - you. And if he is repeatedly calling you it is not because he is trying to punish you, it's because he desperately needs you, and if he gets angry it's not because he's trying to punish you, quite the opposite he probably just doesn't understand why you are hurting him and making him feel so terrible - when he needs you most.
I don't think you're looking at this from the right way around at all, putting in boundaries is not going to work in any way, shape or form. In his terror he will not be able to conform to your boundaries. What you have to do is literally the complete opposite, you have to make him feel safe. Apart from meds, feeling safe is the only thing that is going to reduce his anxiety IMO. He goes to school because he feels safe there, now you have to help him learn that other things are safe to - keep encouraging but don't push too hard or it'll have the opposite effect. You have to listen and learn from him what he can cope with.
I would talk to the Psychiatrist about the possibility of meds, I read with anti d's there's an increased risk of suicide in children so it has to be done by a Psychiatrist I believe but I would definitely consider it as the levels of stress and anxiety he is feeling sound horrific and it could really help him to cope with day to day life, no to mention you. Good luck, read up all you can on ASD to help you understand him and i hope he gets the help he desperately needs.